Resistance against cis-dichlorodiammineplatinum in cultured cells with a high content of metallothionein

Natural products as a means of overcoming cisplatin chemoresistance in bladder cancer

Cisplatin remains an integral part of the treatment for muscle invasive bladder cancer. A large number of patients do not respond to cisplatin-based chemotherapy and efficacious salvage regimens are limited. Immunotherapy has offered a second line of treatment; however, only approximately 20% of patients respond, and molecular subtyping of tumors indicates there may be significant overlap in those patients that respond to cisplatin and those patients that respond to immunotherapy. As such, restoring sensitivity to cisplatin remains a major hurdle to improving patient care. One potential source of compounds for enhancing cisplatin is naturally derived bioactive products such as phytochemicals, flavonoids and others. These compounds can activate a diverse array of different pathways, many of which can directly promote or inhibit cisplatin sensitivity. The purpose of this review is to understand current drug development in the area of natural products and to assess how these compounds may enhance cisplatin treatment in bladder cancer patients.

Cellular mechanisms of zinc dysregulation: a perspective on zinc homeostasis as an etiological factor in the development and progression of breast cancer

Worldwide, breast cancer is the most commonly diagnosed cancer among women and is the leading cause of female cancer deaths. Zinc (Zn) functions as an antioxidant and plays a role in maintaining genomic stability. Zn deficiency results in oxidative DNA damage and increased cancer risk. Studies suggest an inverse association between dietary and plasma Zn levels and the risk for developing breast cancer. In contrast, breast tumor biopsies display significantly higher Zn levels compared with normal tissue. Zn accumulation in tumor tissue also correlates with increased levels of Zn importing proteins. Further, aberrant expression of Zn transporters in tumors correlates with malignancy, suggesting that altered metal homeostasis in the breast could contribute to malignant transformation and the severity of cancer. However, studies have yet to link dysregulated Zn transport and abnormal Zn-dependent functions in breast cancer development. Herein, we summarize studies that address the multi-modal role of Zn dyshomeostasis in breast cancer with respect to the role of Zn in modulating oxidative stress, DNA damage response/repair pathways and cell proliferation/apoptosis, and the relationship to aberrant regulation of Zn transporters. We also compare Zn dysregulation in breast tissue to that of prostate, pancreatic and ovarian cancer where possible.

Melanomas display increased cytoprotection to hypericin-mediated cytotoxicity through the induction of autophagy

Photodynamic therapy (PDT) as a regime for melanoma is of limited success due to factors such as the efficacy of the photosensitizer used, penetration depth and the presence of pigment. We characterised a pigmented and an unpigmented melanoma cell line with respect to their phenotypes. Cell viability was assessed after exposure to hypericin, a UVA-activated photosensitizer. Exposure to 3 microM activated hypericin induced a cytoprotective (autophagic) response from both cell lines. However, the pigmented cells accumulated a large amount of glycogen in their cytoplasm. We hypothesise that the treatment induces an initial cytoprotective response through autophagy, but with increased stress results in a different mode of cell death in pigmented melanoma cells from unpigmented cells. These results indicate that hypericin-PDT could be an adjuvant therapy for melanoma.

Over-expression of metallothionein predicts chemoresistance in breast cancer

Metallothionein (MT) plays a role in fundamental cellular processes such as proliferation, apoptosis and differentiation. We examined MT expression in women with invasive breast ductal carcinoma who underwent mastectomy/lumpectomy without neo-adjuvant treatment. We showed that MT was over-expressed in 87.9% of breast cancer tissues examined, with the mean percentage of positive cells at 30%. There were two patterns of MT expression: predominantly cytoplasmic in 75.9% and nuclear in 24.1% of MT-positive cases. Higher MT scores were associated with poorer histological grade (p = 0.009) but were independent of age, tumour size and oestrogen receptor status. For patients who were treated with adjuvant chemotherapy (cyclophosphamide/methotrexate/5 fluorouracil- or doxorubicin-based regimes), those with high MT expression had a significantly lower recurrence-free survival (p = 0.048), suggesting a role of MT in predicting disease recurrence. Down-regulation of MT in MCF-7 cells by silencing the MT-2A gene (the most abundantly expressed of the 10 known functional MT isoforms) increased chemosensitivity of the cells to doxorubicin. To examine the mechanisms underlying these clinical data, we used siRNAs to decrease MT-2A mRNA expression and protein expression. In MT down-regulated cells challenged with the IC(50) concentration of doxorubicin, we observed a significant reduction in cell viability. Cell cycle analysis also revealed a corresponding increase in apoptosis in the MT down-regulated cells following doxorubicin exposure, showing that down-regulation of MT increased susceptibility to doxorubicin cytotoxicity. The data suggest that MT could be a potential marker of chemoresistance and a molecular therapeutic target.

Tumours from a Cell Strain with a High Content of Metallothionein Show Enhanced Resistance against Cis-Dichlorodiammineplatinum

Cultured cells with a high content of the extremely cysteine-rich protein metallothionein (MT) have previously been shown to exhibit resistance when exposed to otherwise lethal doses of cis-dichlorodiammineplatinum (cis-DDP), chlorambucil or prednimustine, and to have a significant proportion of intracellular drug associated with MT after such treatment. In order to study this protective mechanism in vivo, cells from a MT-rich variant of a murine fibroblast line resistant to cis-DDP and its parent sensitive line with only trace amounts of MT, were injected subcutaneously into nude mice (24 animals in each group), and tumour growth was compared during cis-DDP treatment. Animals in the treatment groups received 3 intravenous doses of either 4 mg/kg or 8 mg/kg of cis-DDP on day 12, 26 and 33 after inoculation of cells, whereas the control groups received saline. The 8 mg/kg dose produced an almost complete growth inhibition of the tumours derived from the parent cells, as well as from the MT-rich variant. However, following the injections with 4 mg/kg of cis-DDP, tumour volume was reduced by approximately 80% in tumours from the parent cells, whereas the tumours from MT-rich cells were almost completely resistant. This study provides for the first time evidence that metallothionein-conferred protection against cis-DDP toxicity also is mediated in tumours in vivo.

The anti-tumour agent, cisplatin, and its clinically ineffective isomer, transplatin, produce unique gene expression profiles in human cells

Cisplatin is a DNA-damaging anti-cancer agent that is widely used to treat a range of tumour types. Despite its clinical success, cisplatin treatment is still associated with a number of dose-limiting toxic side effects. The purpose of this study was to clarify the molecular events that are important in the anti-tumour activity of cisplatin, using gene expression profiling techniques. Currently, our incomplete understanding of this drug's mechanism of action hinders the development of more efficient and less harmful cisplatin-based chemotherapeutics. In this study the effect of cisplatin on gene expression in human foreskin fibroblasts has been investigated using human 19K oligonucleotide microarrays. In addition its clinically inactive isomer, transplatin, was also tested. Dualfluor microarray experiments comparing treated and untreated cells were performed in quadruplicate. Cisplatin treatment was shown to significantly up- or down-regulate a consistent subset of genes. Many of these genes responded similarly to treatment with transplatin, the therapeutically inactive isomer of cisplatin. However, a smaller proportion of these transcripts underwent differential expression changes in response to the two isomers. Some of these genes may constitute part of the DNA damage response induced by cisplatin that is critical for its anti-tumour activity. Ultimately, the identification of gene expression responses unique to clinically active compounds, like cisplatin, could thus greatly benefit the design and development of improved chemotherapeutics.

Modifications of nuclear architecture and chromatin organization in ataxia telangiectasia cells are coupled to changes of gene transcription

Ataxia telangiectasia (AT) is a rare genetic disorder caused by mutations of ATM gene. ATM kinase is a "master controller" of DNA-damage response and signal transducer of external stimuli. The complex role of ATM may explain the pleiotropic phenotype characteristic of AT syndrome, only partially. In our hypothesis, the multi-faceted phenotype of AT patients might depend on specific chromatin reorganization, which then reflects on the cellular transcription. We analyzed three lymphoblastoid cell-lines isolated from AT patients and one healthy control. The three-dimensional reconstruction disclosed marked changes of nuclear morphology and architecture in AT cells. When chromatin condensation was analyzed by differential scanning calorimetry, a remodeling was observed at the level of fiber folding and nucleosome conformation. Despite the structural differences, chromatin did not exhibit modifications of the average acetylation status in comparison to the control. Moreover, AT cells presented significant alterations in the transcription of genes involved in cell-cycle regulation and stress response. In AT3RM cells, the average chromatin decondensation went with the upregulation of c-fos, c-jun, and c-myc and downregulation of metallothioneins, p21 and p53. AT9RM and AT44RM cells were instead characterized by an increased chromatin condensation and presented a different transcription unbalance. Whereas in AT44RM all the considered genes were downregulated, in AT3RM the three oncogenes and metallothioneins were upregulated, but p53 and p21 were downregulated.

Reversal of acquired cisplatin resistance by modulation of metallothionein in transplanted murine tumors

BACKGROUND

The platinum-based chemotherapeutic agent cisplatin is involved in a broad spectrum of activities against human systemic malignancies. However, acquired resistance to cisplatin reduces its clinical efficacy. Elucidation of the molecular basis of cisplatin resistance is required to improve the effectiveness of cisplatin. In the present study, the mechanism of acquired resistance to cisplatin was studied in C3H mice inoculated with MBT-2 murine bladder tumor cells.

METHODS

C3H mice were subcutaneously inoculated with 1.0 x 10(6) MBT-2 cells/mouse on day 0. The mice were given intraperitoneal injections of 10 micro mol/kg cisplatin and subcutaneous injections of 1000 micro mol/kg propargylglycine, an inhibitor of gamma-cystathionase, once a day for 10 consecutive days from day 11 to day 20.

RESULTS

The metallothionein content of the tumors was increased to twice the control level by repeated administration of cisplatin. Co-administration of propargylglycine reduced metallothionein induction in the tumors and markedly enhanced the antitumor activity of cisplatin. In contrast, the glutathione content in the tumors did not change from the control level after cisplatin administration. The platinum accumulation in tumors treated with cisplatin alone was 1.7-fold greater than when propargylglycine was administered concomitantly. The platinum concentrations changed in accordance with the metallothionein contents.

CONCLUSIONS

These observations suggest that metallothionein, but not glutathione or reduced platinum accumulation, might play a role in the acquired resistance to cisplatin of C3H mice inoculated with MBT-2. Moreover, reversal of this resistance might be possible by biochemical modulation of metallothionein.

Copper(II) protects yeast against the toxicity of cisplatin independently of the induction of metallothionein and the inhibition of platinum uptake

We have made the unexpected discovery that copper sulfate protects Saccharomyces cerevisiae from the toxic effects of cisplatin. Addition of copper to the culture medium of yeast cells at concentrations above 0.1 microM significantly reduced the toxicity of cisplatin. Since a high-affinity copper transporter, Ctr1, has been reported to play a major role in the uptake of cisplatin, we examined the effects of copper on the cellular uptake of cisplatin. We found that the cellular concentration of platinum was not significantly affected by treatment of cells with 1 microM copper. It is known that mammalian metallothionein is induced by copper and is involved in acquired resistance to cisplatin. Copper significantly increased the level of mRNA for yeast metallothionein at a concentration that has effectively reduced the toxicity of cisplatin. However, the toxicity of cisplatin in cells with a disrupted gene for ACE1, a factor that regulates transcription of the yeast gene for metallothionein, was also significantly reduced by treatment with copper. These results suggest that copper protects yeast cells from cisplatin toxicity independently of induction of the synthesis of metallothionein and of the inhibition of platinum uptake. Since copper is one of the trace elements that are essential for cell function and since a relatively low concentration of copper (0.1 microM) significantly reduced cisplatin toxicity, it is possible that copper might play an important role in the expression of cisplatin toxicity.

Mechanisms of resistance to cisplatin

The use of cisplatin in cancer chemotherapy is limited by acquired or intrinsic resistance of cells to the drug. Cisplatin enters the cells and its chloride ligands are replaced by water, forming aquated species that react with nucleophilic sites in cellular macromolecules. The presence of the cisplatin adducts in DNA is thought to trigger cell cycle arrest and apoptosis. Knowledge of the mechanism of action of cisplatin has improved our understanding of resistance. Decreased intracellular concentration due to decreased drug uptake, increased reflux or increased inactivation by sulfhydryl molecules such as glutathione can cause resistance to cisplatin. Increased excision of the adducts from DNA by repair pathways or increased lesion bypass can also result in resistance. Finally, altered expression of regulatory proteins involved in signal transduction pathways that control the apoptotic pathway can also affect sensitivity to the drug. An improved understanding of the mechanisms of resistance operative in vivo has identified targets for intervention and may increase the utility of cisplatin for the treatment of cancer.

Induction, regulation, degradation, and biological significance of mammalian metallothioneins

MTs are small cysteine-rich metal-binding proteins found in many species and, although there are differences between them, it is of note that they have a great deal of sequence and structural homology. Mammalian MTs are 61 or 62 amino acid polypeptides containing 20 conserved cysteine residues that underpin the binding of metals. The existence of MT across species is indicative of its biological demand, while the conservation of cysteines indicates that these are undoubtedly central to the function of this protein. Four MT isoforms have been found so far, MT-1, MT-2, MT-3, and MT-4, but these also have subtypes with 17 MT genes identified in man, of which 10 are known to be functional. Different cells express different MT isoforms with varying levels of expression perhaps as a result of the different function of each isoform. Even different metals induce and bind to MTs to different extents. Over 40 years of research into MT have yielded much information on this protein, but have failed to assign to it a definitive biological role. The fact that multiple MT isoforms exist, and the great variety of substances and agents that act as inducers, further complicates the search for the biological role of MTs. This article reviews the current knowledge on the biochemistry, induction, regulation, and degradation of this protein in mammals, with a particular emphasis on human MTs. It also considers the possible biological roles of this protein, which include participation in cell proliferation and apoptosis, homeostasis of essential metals, cellular free radical scavenging, and metal detoxification.

Concurrent expressions of metallothionein, glutathione S-transferase-pi, and P-glycoprotein in colorectal cancers

PURPOSE

Because the status of the inherent drug-resistance of colorectal cancers remains obscure, human colorectal cancers with no neoadjuvant therapy were retrospectively investigated regarding the expression of three drug-resistant proteins: metallothionein, glutathione S-transferase-pi, and P-glycoprotein.

METHODS

Paraffin-embedded tissues of 130 colorectal cancers (Dukes A, 20; B, 49; C, 41; D, 20) obtained by surgical resections from 1982 to 1989 were used. The three proteins were immunostained by the streptavidin-biotin complex method. The immunostaining was judged to be positive if more than 5 percent of cells showed positive staining by use of cell analysis system. The data were compared with clinicopathologic features (Dukes A-D) and patients' prognosis (Dukes AC).

RESULTS

Metallothionein, glutathione S-transferase-pi, and P-glycoprotein were positively expressed in 91 (70 percent), 30 (23 percent), and 98 (75 percent), respectively. A total of 120 (86 percent) expressed at least one drug-resistant protein. No intergroup differences were observed between positive and negative expressions of the proteins and their clinicopathologic features except tumor location. Rectal cancers positively expressed P-glycoprotein and three proteins more frequently. Twenty-six (20 percent), 65 (50 percent), and 21 (16 percent) cancers positively expressed one, two, and three proteins, respectively. The disease-free survival rates of patients with Dukes A through C cancer with positive staining for one, two, and three proteins were 100, 94, and 83 percent (at 1 year); 100, 72, and 51 percent (at 3 years); and 94, 66, and 38 percent (at 5 years), respectively (Kaplan-Meier with log-rank test; P = 0.016). In the multivariate Cox analysis, age, Dukes stage, tumor size, and glutathione S-transferase-pi were independent prognostic factors.

CONCLUSIONS

The patients with concurrent expression of drug-resistant proteins in their cancers had worse prognoses. Examining drug-resistant proteins in colorectal cancers may be useful in selecting adjuvant chemotherapy and in predicting prognosis more accurately.

CROP/Luc7A, a novel serine/arginine-rich nuclear protein, isolated from cisplatin-resistant cell line

A novel putative SR protein, designated cisplatin resistance-associated overexpressed protein (CROP), has been cloned from cisplatin-resistant cell lines by differential display. The N-half of the deduced amino acid sequence of 432 amino acids of CROP contains cysteine/histidine motifs and leucine zipper-like repeats. The C-half consists mostly of charged and polar amino acids: arginine (58 residues or 25%), glutamate (36 residues or 16%), serine (35 residues or 15%), lysine (30 residues, 13%), and aspartate (20 residues or 9%). The C-half is extremely hydrophilic and comprises domains rich in lysine and glutamate residues, rich in alternating arginine and glutamate residues, and rich in arginine and serine residues. The arginine/serine-rich domain is dominated by a series of 8 amino acid imperfect repetitive motif (consensus sequence, Ser-Arg-Ser-Arg-Asp/Glu-Arg-Arg-Arg), which has been found in RNA splicing factors. The RNase protection assay and Western blotting analysis indicate that the expression of CROP is about 2-3-fold higher in mRNA and protein levels in cisplatin-resistant ACHN/CDDP cells than in host ACHN cells. CROP is the human homologue of yeast Luc7p, which is supposed to be involved in 5'-splice site recognition and is essential for vegetative growth.

HeLa cell transformants overproducing mouse metallothionein show in vivo resistance to cis-platinum in nude mice

Plasmid pSV2MT-I encoding mouse metallothionein-I (MT-I) designed to be expressed under the control of an SV40 promoter was introduced into human HeLa S3 cells. Several transformants (HeLa/MTH) carrying multi-copies of mouse MT-I cDNA in their genomes were isolated. These transformants produced 4 to 20-fold larger amounts of MT than their parent cells. The MT levels in HeLa/MTH were well correlated with the extent of resistance to cadmium, but not with that to cis-platinum (cis-DDP) in vitro. To study the role of MT in resistance to cis-DDP in vivo, nude mice were inoculated subcutaneously with two independent HeLa/MTH clones. MT levels in these tumors were about 3-fold higher than those in the parental cells. The growth of tumors derived from either HeLa/MTH clone was not inhibited in the presence of 15 micromol/kg of cis-DDP, which completely inhibited the growth of tumors derived from the parental HeLa cells. These data strongly suggest that the elevated level of MT confers resistance to cis-DDP in vivo but not in vitro. Thus, the results of this study indicate that in vitro determinations of the influence of MT on cis-DDP resistance may underestimate its importance in in vivo situations.

Induction of drug resistance to gold sodium thiomalate in a monocyte cell line, THP-1

The expression of metallothionein, an intracellular heavy-metal-binding protein, and p-glycoprotein, an energy-dependent drug efflux pump, was examined to study the mechanism of cell resistance to gold sodium thiomalate (GST). THP-1, one of the monocyte-derived cell lines, was cultured for 6 months and resistance to 25 microg/ml of GST (GST-resistant cells) was thus induced. The GST-resistant cells were then cultured with bucillamine to examine the presence of cross-resistance. The intracellular GST concentration was examined by flameless atomic absorption spectroscopy. The cell viability was determined by the uptake of 3-4,5 dimethylthiazole-2,5 diphenyl tetrazolium bromide (MTT). The expression of p-glycoprotein was detected by Western blotting using monoclonal anti-p-glycoprotein antibody. The expression of metallothionein was detected using the indirect immunofluorescence technique. GST-resistant cells did not show any cross-resistance to bucillamine. The rate of cytoplasmic GST accumulation decreased in the GST-resistant cells, while the rate of GST efflux also decreased. The expression of p-glycoprotein in the GST-resistant cells was not significantly different from that in the cells not treated with GST. On the other hand, the GST-resistant cells showed a higher expression of metallothionein than cells not treated with GST. These findings suggest that the induced resistance to GST might partly be due to an induction of metallothionein.

Reduction of cellular cisplatin resistance by hyperthermia--a review

Resistance to cisplatin (cDDP) is a major limitation to its clinical effectiveness. Review of literature data indicates that cDDP resistance is a multifactorial phenomenon. This provides an explanation why attempts to reverse or circumvent resistance using cDDP-analogues or combination therapy with modulators of specific resistance mechanisms have had limited success so far. It therefore provides a rationale to use hyperthermia, an agent with pleiotropic effects on cells, in trying to modulate cDDP resistance. In this review the effects of hyperthermia on cDDP cytotoxicity and resistance as well as underlying mechanisms are discussed. Hyperthermia is found to be a powerful modulator of cDDP cytotoxicity, both in sensitive and resistant cells. Relatively high heat doses (60 min 43 degrees C) seem to specifically interfere with cDDP resistance. The mechanism of interaction has not been fully elucidated so far, but seems to consist of multiple (simultaneous) effects on drug accumulation, adduct-formation and -repair. This may explain why hyperthermia seems to be so effective in increasing cDDP cytotoxicity, irrespective of the presence of resistance mechanisms. Therefore, the combination of hyperthermia and cDDP deserves further attention.

Metallothionein in physiological and physiopathological processes

The multipurpose nature of MT that we have presented in this review has drawn attention from many different fields of research: biochemistry, molecular biology, toxicology, pharmacology, etc. In recent years, considerable advances have been made concerning the regulation of MT genes by metals. Little, however, is known at the molecular level about the mechanisms of MT induction by nonmetallic inducers such as growth factors. This is of particular interest since MT is highly expressed during liver regeneration, an event orchestrated by a series of growth stimulators and inhibitors. The significance of the nuclear distribution of MT in growing cells and what controls its translocation are questions that remain unanswered at the present time. The possibility that MT could participate in a DNA synthesis-related process through donation or abstraction of Zn to and from transcription factors has been inferred from in vitro studies. Such transfer mechanisms, however, have yet to be confirmed in vivo. Overexpression of MT is often accompanied by increased resistance towards a variety of alkylating agents and chemotherapeutic drugs. The mechanisms by which MT protects cells against these agents may depend on their distinct mode of toxic action. For some, MT cysteines can be the target of the direct attack from the parent compound. For others such as N-methyl-N-nitroso compounds, MT cysteines may serve as a sink for the reactive oxygen species now known to be derived from their metabolism. In either case, a primary consequence of such interactions is the release of the metals initially bound to MT. Therefore, the metal composition of MT appears to be an important factor to consider in determining the overall effect of MT in the resistance process.

Multifactorial drug-resistance phenomenon in acute leukemias: impact of P170-MDR1, LRP56 protein, glutathione-transferases and metallothionein systems on clinical outcome

The multidrug resistance phenomenon can be observed in cases which do not express the P170 protein and these cases are suspected as having activated different resistance phenomena. Four phenomena were studied at the time of diagnosis in a series of 35 lymphoblastic and 25 myeloblastic acute (de novo) leukemias, by an immunocytochemical method. Two energetic drug transport processes were investigated: the classical MDR/P170 and the P110/LRP56 proteins, and two physiological detoxifying activities such as the glutathione transferases (GST alpha, mu, pi) and the metallothioneins (Mts). The results demonstrate that these phenomena are independent but their synergic activity can increase their impact on the outcome. P110/LRP56 positive cases demonstrated 48.8% complete remission (CR) rate compared to 71.4% for negative tests. When P170 and P110 were both positive or negative, the CR rates were 27.3% and 81.8% respectively (p = 0.0120), and survival curves were also different (p = 0.030). The CR rate in AML or ALL is weakly affected by GST pi, alpha or mu but relapses are more frequently observed for Positive-GST pi ALL (p = 0.0658). Patients with both P170 and GST pi positive reactions had a 53.3% CR rate compared to 78.9% for both negative reactions. Survival curves for these two groups were different. The CR rate in AMl was 100% for Mts positive and 43.7% for negative cases (p = 0.050), however the median survival was totally different for these two groups (p = 0.046). CR rates were 26.6% for patients who were P170 positive and Mts negative compared to 100% for P170 negative and Mts positive (p = 0.038) patients. Survival curves were also different (p = 0.0510). We conclude that these four mechanisms induce an independent drug resistance but their synergic action increase their impact on the outcome. The metallothioneins seem to have a major impact on the drug resistance phenomenon and its effect should be investigated with high priority, in the light of these results.

Immunohistochemical study of metallothionein in pancreatic carcinomas

Metallothioneins are a family of intracellular metalloproteins that have been thought to be involved in anticancer drug resistance. However, the role of metallothioneins in pancreatic cancer has not been investigated in detail. The immunohistochemical localization of metallothionein was examined in normal human adult pancreas tissue and in 75 pancreatic duct cell carcinomas, using monoclonal anti-metallothionein antibody. Furthermore, in vitro studies on the sensitivity of pancreatic cancer to cisplatin were performed in 10 cases of pancreatic carcinoma. Metallothionein staining was weakly positive in the acinar and islet cells and intralobular ducts but was negative in the large pancreatic ducts. In pancreatic carcinomas, metallothionein staining was diffusely positive in 6 (8%), focally positive in 25 (33%) and negative in 44 (59%) of the 75 pancreatic carcinomas. The expression of metallothioneins in pancreatic tumors was related to metastasis, poor prognosis and poor histological grading (poorer glandular differentiation and nuclear anaplasia). The in vitro study of tumor sensitivity to cisplatin showed no significant correlation between metallothionein expression and resistance to cisplatin. Metallothionein-positive pancreatic carcinoma will be potentially highly malignant or acquire an enhanced ability to produce metallothioneins as the malignant potential increases. The expression of metallothionein could be a prognostic indicator in pancreatic carcinomas.

Effect of preinduction of metallothionein synthesis on clastogenicity of anticancer drugs in mice

The effect of pretreatment with metallothionein (MT) inducers (bismuth nitrate or zinc chloride) on clastogenicity of anticancer drugs was investigated. Bismuth nitrate (50 mumol/kg) or zinc chloride (400 mumol/kg) was administered s.c. to mice once a day for two days prior to treatment with 3.3 mumol/kg of cis-diamminedichloroplatinum(II) (cis-DDP), 3.4 mumol/kg of adriamycin (ADR), 72 mumol/kg of cyclophosphamide (CPA) or 0.41 mumol/kg of L-phenylalanine mustard (L-PAM). The frequency of occurrence of erythrocytes with micronuclei in bone marrow was increased by each anticancer drug at 24 h after treatment. Micronucleus formation was significantly prevented by pretreatment with either bismuth nitrate or zinc chloride. MT concentration in bone marrow cells of mice at the time of treatment with anticancer drugs increased to 2- and 3.5-fold by pretreatment with bismuth nitrate and zinc chloride, respectively. These results indicate that MT induction in bone marrow cells effectively prevents micronucleus induction of anticancer drugs.

Interaction of cis- and trans-diamminedichloroplatinum with metallothionein in vivo

The properties of platinum (II) complexes to induce the biosynthesis of metallothionein (MT) were investigated in rabbits following injections of K2PtCl4, cis and trans isomers of DDP (diammine-dichloroplatinum). It was demonstrated that cis-DDP has an ability to induce MT specifically in the liver, whereas trans-DDP appears to be unable to stimulate the biosynthesis of MT in either the liver or the kidneys. In contrast, K2PtCl4 is effective to elevate the MT level in both tissues. However, all of these platinum complexes are rather poor stimulators for MT biosynthesis compared to cadmium and zinc compounds. Preinjection with Zn(NO3)2 significantly enhances the amount of Pt associated with the MT fractions compared to that resulting from injections with either cis- or trans-DDP without Zn(NO3)2 pretreatment. Metallothionein containing Pt was purified and identified from the liver and kidneys of rabbits after preinjections with Zn(NO3)2 followed by repeated injections of cis-DDP and trans-DDP, respectively. It was found for the first time that a relatively higher degree of Pt was associated with MT fractions in the case of trans-DDP treatment than that of cis-DDP injection. On this basis, the role of MT was discussed in relation to its involvement in the metabolism of cis-DDP, and the difference of the antitumor activity and toxicity between cis- and trans-DDP.

Covalent sequestration of melphalan by metallothionein and selective alkylation of cysteines

Rabbit liver metallothionein-2 is shown to form covalent bonds with the anticancer agent melphalan, in support of the hypothesis that covalent sequestration by metallothionein constitutes one mechanism for the cross-resistance acquired by cancer patients to therapeutic alkylating agents. Among 20 cysteines in the 2-domain protein, 89% of the first alkylation reaction occurs with 2 that cochelate a zinc cation in the carboxy domain. Computer-supported docking studies indicate a favorable binding site for melphalan near these cysteine sulfhydryl groups. Although folded metallothionein-2 is resistant to trypsin cleavage, alkylation by 1 mol of melphalan allows cleavage by trypsin between the two globular domains.

Combination therapy with cisplatin and nifedipine inducing apoptosis in multidrug-resistant human glioblastoma cells

The authors found that multidrug-resistant human glioblastoma GB-1 cells demonstrated significantly more resistance to cisplatin than did nondrug-resistant human glioblastoma U87-MG cells (p < 0.1). They therefore attempted to determine whether calcium channel blockers enhance the antitumor activity of cisplatin against GB-1 cells. Nifedipine, a dihydropyridine calcium channel blocker, significantly enhanced the antitumor effect of cisplatin on GB-1 cells (p < 0.05). In the absence of normal extracellular Ca++, nifedipine enhanced the cytotoxicity of cisplatin. In addition, the antitumor activity of combined cisplatin and nifedipine was inhibited both by actinomycin D and cycloheximide, suggesting that such activity is dependent upon new RNA and protein synthesis. Surprisingly, DNA fragmentation assay demonstrated that synergism between cisplatin and nifedipine resulted in apoptosis (programmed cell death) at a relatively low concentration of cisplatin, which when tested alone did not induce apoptosis. In addition, it was demonstrated that nuclei from GB-1 cells lacked a Ca(++)-dependent endonuclease that degrades chromatin into nucleosomes and that calcium ionophore A 23187 did not decrease the viability of GB-1 cells. The above findings suggest the hypothesis that the noncytotoxic agent nifedipine synergistically enhances the antitumor effect of cisplatin on multidrug-resistant GB-1 cells lacking Ca(++)-dependent endonuclease, and subsequently induces apoptosis via its interaction with an as yet uncharacterized functional site other than the calcium channel on GB-1 cells.

Combination therapy with cisplatin and nifedipine induces apoptosis in cisplatin-sensitive and cisplatin-resistant human glioblastoma cells

We attempted to determine whether calcium channel blockers (CCBs) enhance the anti-tumour activity of cis-diamminedichloroplatinum (cisplatin) against both cisplatin-sensitive human glioblastoma U87 MG cells and cisplatin-resistant U87-MG-CR cells, the latter of which we developed for resistance to cisplatin. Nifedipine, a dihydropyridine class CCB, significantly enhanced the anti-tumour effect of cisplatin on these two cell types in vitro and in vivo. Our findings also indicated that, in the absence of normal extracellular Ca2+ nifedipine was capable of enhancing the cytotoxicity of cisplatin. In addition, this anti-tumour activity was partially inhibited by actinomycin D and cycloheximide, suggesting that it is possibly dependent upon new RNA and protein synthesis. Interestingly, ultrastructural analysis, DNA fragmentation assay and cell cycle analysis demonstrated that synergism between cisplatin and nifedipine results in apoptosis (programmed cell death) at a relatively low concentration of cisplatin, which when tested alone did not induce apoptosis. Furthermore, we demonstrated that nuclei from these cells lack a Ca(2+)-dependent endonuclease that degrade chromatin in the linker region between nucleosomes. In conclusion, our studies suggest that the non-cytotoxic agent nifedipine is able to synergistically enhance the anti-tumour effects of cisplatin on U87-MG and U87-MG-CR cells lacking a Ca(2+)-dependent endonuclease and subsequently to induce apoptosis via interaction of nifedipine with an as yet uncharacterised functional site other than a calcium channel on target cells.

An M(r) 7-kDa membrane protein overexpressed in human multidrug-resistant ovarian cancer cells

We have developed a monoclonal antibody (designated 1D7) which recognizes an M(r) 7-kDa plasma membrane protein overexpressed in ovarian MDR cancer cells. The expression of the M(r) 7-kDa protein in various human multidrug-resistant and drug-sensitive cell lines was analysed by Western blot and flow cytometry methods. The small molecular weight protein was overexpressed in the human ovarian carcinoma cell line, SKVLB which was selected for vinblastine resistance from SKOV3 cells and in OVCAR 4/ADR100 and OVCAR 4/VBL200 which were generated from NIH:OVCAR4 by stepwise selection against adriamycin and vinblastine, respectively. Only a minor amount of the M(r) 7-kDa protein was found in the parent cell line, SKOV3. It was not found in other drug-resistant human cell lines such as the vinblastine-resistant CEM cells (CEM/VLB300), the intrinsic MDR colon cell line HCT15 and the human MDR breast cancer cell line, MCF7/AdrVp. 1D7 specifically inhibited the proliferation of the resistant cells. Our results suggest that the expression of the M(r) 7-kDa protein on the plasma membrane of ovarian MDR cancer cells may be involved in a mechanism related to the proliferation of the drug resistant cancer cells.

Exposure of human ovarian carcinoma to cisplatin transiently sensitizes the tumor cells for liposome-mediated gene transfer

Human ovarian carcinoma cells (line 2008) grown as subcutaneous solid tumor in the severe combined immunodeficient mouse can be transfected by directly injecting a plasmid DNA-liposome complex into the tumor (in situ lipofection). The level of reporter gene expression in the tumor cells was significantly elevated if the animal received a single i.p. injection of cisplatin 1 week before the lipofection. Sensitization of the tumor for lipofection peaked 1 week after cisplatin injection and declined thereafter. Cells exposed to low concentration of cisplatin in vitro for four to five doubling times also showed elevated sensitivity for lipofection in vitro. Cisplatin was the only anticancer drug tested that exhibited this activity. These results suggest a sequential combinational gene therapy protocol with cisplatin for the ovarian carcinoma.

Absence of immunohistochemical metallothionein staining in bladder tumor specimens predicts response to neoadjuvant cisplatin, methotrexate and vinblastine chemotherapy

Pre-chemotherapy bladder tumor tissue was retrospectively analyzed in 21 patients treated with neoadjuvant cisplatin, methrotrexate and vinblastine. Immunohistochemical staining for 2 proposed mediators of drug resistance, p-glycoprotein and metallothionein, was performed and compared to chemotherapeutic response and survival. There was no significant relationship between staining for p-glycoprotein and the response to chemotherapy or survival. Patients with no detectable staining for metallothionein were statistically more likely to sustain a complete pathological response (p = 0.029) after chemotherapy than those with any detectable staining. No relationship between metallothionein immunostaining and survival was apparent. This study offers further evidence linking metallothionein in tumor resistance to cisplatin containing chemotherapy regimens.

Metallothionein expression is correlated with cisplatin resistance in transitional cell carcinoma of the urinary tract

Metallothioneins (MTs) are small cysteine-rich, metal-binding proteins involved in resistance to heavy-metal toxicity, and are known to bind cisplatin. Several experiments suggest possible involvement of MT in cellular resistance to cisplatin. To investigate the relationship between MT expression and cisplatin resistance in urinary tract transitional cell carcinoma (TCC), immunohistochemical staining for MT was performed in 31 untreated TCCs of the urinary tract. The results were compared with the sensitivity of the tumors to cisplatin as assessed by the microtiter succinate dehydrogenase inhibition (mSDI) test. Variable MT expression was observed in all 31 TCCs, but there was no specific correlation between histopathological parameters and MT expression. Fourteen (87.5%) of the 16 TCCs with less than 10% of their tumor cells positive for MT were sensitive to cisplatin. On the other hand, 6 (75.0%) of the 8 TCCs with MT expression by more than 30% of their tumor cells were resistant to cisplatin, and there was a significant correlation between MT expression and cisplatin resistance (p < 0.01). These results suggest the possible involvement of MT in the intrinsic cisplatin resistance of urinary tract TCC and that immunohistochemical investigation of MT may be useful for predicting the response of these tumors to cisplatin therapy.

Basal metallothionein in tumors: widespread presence of apoprotein

A survey has been conducted of solid and ascites tumors from mice and solid tumors in rats for the presence of metallothionein or metallothionein-like protein. In most tumors, a positive identification was made on the basis of Sephadex G-75 and HPLC-DEAE chromatography followed by competitive radioimmunoassay for metallothionein. Apometallothionein was revealed in a number of tumors for the first time by comparing the Sephadex G-75 chromatographic profiles of Zn in native cytosol and Cd in cytosol incubated briefly with CdCl2 to saturate free binding sites on the protein before Sephadex G-75 chromatography. In two cases unsaturation of metallothionein was correlated with a lack of zinc in the ascites fluid which supplies the tumor with zinc.

Increased DNA-repair capacity and the modulation of 2 proteins in and metallothionein overexpressing Chinese hamster cell line

Elevated intracellular levels of metallothionein have been associated with resistance to the cytotoxic effects of some alkylating agents. In order to study the mechanisms responsible for this resistance, we used a pair of CHO cell lines consisting of normal K1-2 cells and their derivative K1-2MT, which overexpresses the human metallothionein II-A gene (Lohrer et al., 1989). K1-2MT cells were found to be resistant to cadmium chloride and the alkylating agents N-methyl-N'-nitro-N- nitrosoguanidine (MNNG), but resistance did not extend to the alkylating agent, 1,3-bis(2-chloroethyl)- 1-nitrosourea, nor to adriamycin, an inhibitor of DNA synthesis. The DNA damage caused by MNNG, was only marginally less in resistant cells compared with the parental cell line, thus excluding drug scavenging as a possible mechanism for resistance. Also, glutathione S-transferases (GSTs) were present at equal levels in both cell lines (acidic and basic type GST) or slightly reduced in drug resistant K1-2MT cells (neutral type GST), thereby ruling out metabolic inactivation of the alkylating agents. However, the drug resistant phenotype was accompanied by a more efficient block of DNA synthesis after MNNG treatment and by a 3-h delay in the G2 phase of the cell cycle. Using two-dimensional gel electrophoresis of total protein extracts, we identified a 24-kDa protein (MIP1), which is only present in the resistant K1-2MT cells, and a 23.5-kDa protein (MIP2) which is 2-3 times over-synthesized in K1-2MT cells. MNNG treatment reduced the level of both proteins MIP1 and MIP2. These results suggest that the proteins MIP1 and possibly MIP2 may be responsible for the alkylating agent resistant phenotype and are probably modulated by the human metallothionein II-A protein.

Effect of pharmacologic doses of zinc on the therapeutic index of brain tumor chemotherapy with carmustine

To evaluate the potential differential effect of pretreatment with pharmacologic doses of the trace element zinc on the chemosensitivity of glioma cells and bone marrow cells for carmustine (BCNU), we performed in vitro and in vivo studies of zinc toxicity as well as of the combined treatment with zinc and the anticancer drug. We studied the in vitro effects on established human and rat glioma cell lines using a microcolorimetric growth assay and on murine bone marrow using a clonogenic assay for committed progenitor cells of the granulocyte-monocyte lineage. Zinc exposures of up to 100 microM for 120 h did not influence the growth of six of seven human glioma cell lines. Only U87MG demonstrated statistically significant toxicity during high zinc exposure (100 microM over 120 h). Dose-response growth curves generated for BCNU did not show protection against the anticancer agents by a 48-h pretreatment with different zinc concentrations. The clonogenic capacity of bone marrow cells was slightly reduced by in vitro culture for 24 and 48 h. Although this effect appeared to be more prominent in the presence of zinc supplementation, overall a statistically significant inhibition was seen only after exposure to a concentration of 100 microM zinc over 48 h. As compared with chemotherapy alone, in vitro pretreatment with 50 microM zinc over 48 h followed by chemotherapy resulted in an increased number of colony-forming unit-granulocyte monocyte (CFU-GM): CFU-GM increased by a factor of 2 for BCNU (60 microM x 2 h). This statistically significant in vitro chemoprotection would translate into a dose-protection factor of 1.5, i.e., for the same level of myelosuppression, zinc pretreatment would allow administration of a 50% increased dose of BCNU. The in vivo studies were performed in an s.c. xenograft model of the human glioma cell line U87MG in athymic mice. The maximal tolerable pretreatment with zinc was determined to be a 10-day course of daily i.p. injections of 10 mg/kg ZnCl2. The subsequent i.p. administration of the dose lethal to 10% of the mice (LD10) and of a 1.5 x LD10 dose of BCNU resulted in less bone marrow toxicity in pretreated animals than in non-zinc-pretreated mice as determined in a CFU-GM assay. Glioma colony-forming efficiency (CFE) assays, on the other hand, did not show any zinc-related difference in the BCNU sensitivity of U87MG.(ABSTRACT TRUNCATED AT 400 WORDS)

The characterization of two human cervical carcinoma HeLa sublines resistant to cisplatin

Human cervical carcinoma HeLa cells were made resistant to cisplatin by one of two schedules; "acute" (cells repeatedly treated with cisplatin for 1 h in serum-free medium--CA cells) or "continuous" (cells treated repeatedly for 24 h in complete medium--CK cells). The sensitivity of CA and CK sublines to cisplatin and various other antineoplastic drugs was determined by the modified MTT staining procedure. The possible involvement of glutathione (GSH), glutathione S-transferases (GST) and metallothioneins (MT) in cisplatin resistance was examined. The results show that acutely treated CA cells became more resistant to cisplatin than CK cells. The resistance to cisplatin does not involve either glutathione or glutathione transferase. The increased levels of metallothioneins might be involved in the development of resistance. The sensitivity of CA and CK sublines to the selected drugs was different from that of the parent cells. Both sublines became cross-resistant to vincristine and methotrexate, but only CA cells exhibited cross resistance to etoposide doxorubicin and 5-fluorouracil. Thus, the development of resistance to cisplatin is a consequence of numerous intracellular alterations that are reflected in cell response to a variety of anticancer drugs. The response depends on the schedule of resistance development and on the nature of the secondary agent.

Reduction of drug accumulation in cisplatin-resistant variants of human prostatic cancer PC-3 cell line

We have isolated cis-diamminedichloroplatinum (II) (CDDP)-resistant variants, P/CDP4 and P/CDP5, from human prostatic cancer PC-3 cells after a stepwise exposure to CDDP. P/CDP4 and P/CDP5 showed 11-fold and 23-fold higher resistance to CDDP than did PC-3. P/CDP5 was cross-resistant to carboplatin, mitomycin C, etoposide, m-AMSA, bleomycin and UV irradiation. Alkaline elution of DNA showed an increased amount of DNA interstrand cross-links in PC-3 but not in P/CDP5 when PC-3 and P/CDP5 were cultured with CDDP. Flameless atomic absorption spectrophotometry revealed that intracellular accumulation of CDDP in P/CDP4 and P/CDP5 was decreased to 18 to 34% and 9 to 18% of that of PC-3, respectively, when PC-3 and its CDDP-resistant counterparts were incubated with 5 and 10 micrograms./ml. of CDDP for 24 hours. These data suggest that decreased drug accumulation is involved in the development of CDDP-resistance in the PC-3 cell line.

Metallothionein in testicular germ cell tumors and drug resistance. Clinical correlation

BACKGROUND

Metallothioneins (MT) are endogenous metalloproteins involved in the homeostasis of essential metals and detoxification of toxic metals. Some recent experimental studies suggested tumor resistance to cisdiamminedichloroplatin may be associated with overexpression of MT in the tumor.

METHODS

The presence of MT in 33 primary testicular germ cell tumor specimens was assessed immunohistochemically using a rabbit polyclonal rat liver MT antibody that cross-reacted with human MT. The data were correlated with the patients' clinical course.

RESULTS

Seminomas stained weakly or not at all for MT, regardless of the clinical stage. Most nonseminomas stained heavily for MT. The more advanced staged nonseminomas tended to stain more heavily for MT.

CONCLUSIONS

In view of the considerable experimental evidence as well as some inferential clinical data involving MT in cis-diamminedichloroplatin resistance, there appears to be a role for MT in cis-diamminedichloroplatin resistance in germ cell tumors. Further studies to elucidate the role of MT in germ cell tumor chemoresistance are warranted.

Metallothionein and anticancer agents: the role of metallothionein in cancer chemotherapy

Metallothioneins (MTs) are intracellular proteins containing the highest amount of thiol groups within the cytoplasm. These thiol groups are able to bind several cytotoxic agents, such as platinum compounds and alkylating agents. Increased levels of MT are one mechanism of resistance to these anticancer drugs, as intracytoplasmic binding of MT prevents the active molecules from reaching their target, the intranuclear DNA of tumor cells. MT synthesis can easily be induced by physiologic heavy metals such as zinc and copper. Pharmacological modulation of MT levels has been used to increase the MT pool in normal tissues and decrease their susceptibility to the toxicity of anticancer drugs. In the case of tumors arising in the brain, where the inducibility of MT synthesis is low, this approach would allow protection of normal tissues without decreasing the antitumor activity of the cytotoxic agents. The interaction of MT with cytotoxic agents is not limited to covalent binding. A correlation between MT synthesis and amplification of oncogenes such as ras has been reported. Furthermore, the cytotoxic drugs are bound by MT after competition with zinc and copper; these metals are cofactors of numerous metalloenzymes, some of which are involved in the metabolism of nucleic acids. Competitive displacement of these metals might modify nucleic acid metabolism and influence cellular proliferation. On the other hand, increased MT levels could provide a zinc cofactor reserve that increases the cell's reparative potential when faced by DNA damage by cytotoxic agents.(ABSTRACT TRUNCATED AT 250 WORDS)

Platinum resistance: laboratory findings and clinical implications

Platinum compounds are universally recognized as one of the most important classes of chemotherapeutic agents for the treatment of cancer. Emergence of resistance to cisplatin has appeared, however, to be a major prognostic factor of adverse outcome in the otherwise most sensitive of malignancies: testicular and ovarian cancers. After a decade of testing both systemic and regional dose-intensification of cisplatin and its analog carboplatin--which is more amenable to dose escalation with cytokines and bone marrow progenitor cell support--a plateau is apparent even in sensitive tumor types beyond which additional dose escalations do not appreciably increase response. Laboratory work searching for causes of intrinsic and acquired resistance, providing early indication of drug sensitivity, and developing strategies for restoring or overcoming resistance is ongoing and is guiding clinical studies and drug development. Causes of cellular resistance to platinums are complex and include decreased drug accumulation, increased detoxification, increased repair of DNA-platinum adducts, and increased tolerance of DNA lesions. Clinical trials are already ongoing regarding strategies involving protection of specific toxicities, decreasing intracellular glutathione (by buthionine sulfoximine), decreasing DNA repair, and introducing new analogs that are able to overcome certain forms of platinum resistance.

Transient cisplatin-resistant murine fibrosarcoma cell characterized by increased metallothionein content

Cisplatin-resistant mouse fibrosarcoma cells, SSK-R, were isolated after short and low-dose drug treatment of the sensitive SSK cells in vitro. These SSK-R sublines exhibit up to sevenfold cisplatin resistance and are characterized mainly by an increased metallothionein content. Loss of drug resistance after about 140-180 cell divisions in drug-free medium coincides with loss of metallothionein content. The glutathione level is the same in the sensitive and resistant sublines; inhibition of glutathione synthesis by buthionine sulphoximine enhances the sensitivity in both cells lines by a factor of 2.7. The resistant sublines are not cross-resistant to radiation; a radiation exposure followed immediately by cisplatin treatment results in an additive effect. The cellular cisplatin content is slightly reduced in SSK-R2 cells and it remains at this level also upon loss of drug sensitivity.

Distinct P-glycoprotein expression in two subclones simultaneously selected from a human colon carcinoma cell line by cis-diamminedichloroplatinum (II)

Two drug-resistant sublines, CP2.0 and RT, were simultaneously selected by cis-diamminedichloroplatinum (CDDP) from the human colon carcinoma cell line LoVo by the conventional method of continuous drug exposure. The 2 sublines differed in morphology, growth kinetics and pattern of gene expression. Genetic signature analysis indicated that the lines were independent subclones but that both arose from LoVo. These sublines were maintained in a growth medium containing 2.0 micrograms/ml CDDP. However, CP2.0 cells were 3 times more resistant to CDDP than were RT cells. Although both were cross-resistant to mustargen and 5-fluorouracil, only CP2.0 was resistant to Adriamycin and vincristine. Western-blot analysis, immunocytochemical staining and in vitro phosphorylation experiments indicated that the level of P-glycoprotein was significantly elevated in CP2.0 but not in RT. Despite the differences between these sublines, they possess similar CDDP-resistance mechanisms, including decreased intracellular CDDP accumulation, elevated levels of glutathione and metallothionein-like proteins, increased glutathione transferase-pi mRNA, and enhanced susceptibility to CDDP cytotoxicity after treatment with DL-buthionine-[S,R]-sulfoximine. Nevertheless, our results suggest that, in certain tumor types, P-glycoprotein-mediated multi-drug resistance and CDDP-resistance phenotypes can coexist in cells with primary resistance to CDDP.

Increased sensitivity of a Chinese hamster ovary cell line to alkylating agents after overexpression of the human metallothionein II-A gene

The elevation of intracellular levels of metallothionein has been associated with the development of resistance to the cytotoxic effects of some alkylating agents. In order to study the mechanisms underlying metallothionein associated drug resistance we transfected the alkylating agent sensitive CHO cell line MMC-1 (Robson et al., 1985) with an episomally replicating plasmid coding for the human metallothionein II-A gene. Two transfectants were isolated which carried about 10 copies of the plasmid. The basal expression of metallothionein was increased from undetectable levels in the recipient cell line MMC-1 to between 22 and 26 micrograms metallothionein per 10(7) cells in the transfectants. Treatment with cadmium salts increased metallothionein levels a further 1.7-fold. Cytotoxicity assays revealed that MTII-A transfection increased the sensitivity of the parental MMC-1 cells to N-methyl-N'-nitro-N-nitrosoguanidine and N-nitroso-N-methylurea. These results suggest that metallothionein does not act as a simple scavenger of alkylating agents, but interacts with unknown factor(s) in host cells.